emerson data-center-cooling_24644-r09-10.pdf

7/25/2019 Emerson data-center-cooling_24644-R09-10.pdf

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A White Paper from the Expertsin Business-Critical Continuity

Precision versus Comfort CoolingChoosing a Cooling System to Support Business-Critical IT Environments


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Executive Summary

Ensuring efcient operation without compromising reliability is essential to the success ofcompanies that depend on achieving business-critical continuity. Unfortunately, whetherdue to rapid, unanticipated growth or a general lack of awareness with regard to thecooling requirements of sensitive electronic equipment, some data center professionals

particularly managers of small and/or remote facilities are not able to provide adequateattention to their data centers infrastucture system design.

Many data center managers end up trying to use comfort cooling solutions to satisfytheir environmental control needs, as these systems can mean simplied and economicaldeployment up front. However, because they are not designed to meet the unique needs ofcritical IT equipment, reliance on comfort cooling systems can result in an increased risk forpremature system failures as well as grossly inated operating costs. In contrast, precisioncooling systems are designed specically to meet the needs of dense electrical heat loads.

The decision to accept the increased risk of comfort cooling systems is usually based onthe lower initial costs. However, precision cooling systems actually represent the most

cost-effective solutions when total cost of ownership is evaluated. Finally, data centers thatincorporate high density cooling design into their expansion or renovation plans will benetfrom scalable IT capacity and reduce the risk of IT downtime.

Consider the following:

On average, Total Cost of Ownership (TCO) for comfort cooling systems is signicantlyhigher up to 60 percent than that of precision cooling systems over the life of theequipment.

The cost of downtime also is rapidly increasing as businesses rely more heavily on businesscritical IT systems. The failure of critical data center systems means a loss of service andcustomer goodwill that can translate to losses of up to $50,000 per hour on average.

While precision cooling systems enable IT managers to achieve the highest levels ofequipment reliability and performance efciency, it is important to remember that thereliability and cost effectiveness of any cooling system is largely dependent upon properservice and maintenance. Therefore, leveraging a team of factory-trained technicians whohave an extensive knowledge of how to maintain mission-critical cooling systems is key toachieving high reliability and the lowest possible TCO.


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Precision vs. Comfort Cooling

In small or remote facilities, the carefulconsideration of equipment essentialto reliable operation is a step frequentlyoverlooked in favor of other aspects of the

planning process. As a result, areas such asenvironmental control, power continuityand monitoring of critical computer supportsystems often receive far less attention thandecisions about servers, operating systemsand other network-level components thatmake up a critical IT infrastructure. However,achieving business-critical continuity is asdependent on power and environmentalsupport systems as on advanced networkingequipment.

The high sensitivity of electroniccomponents in data center and networkingenvironments means that temperature,humidity, air movement and air cleanlinessmust be kept consistent and within speciclimits to prevent premature equipmentfailures and costly downtime. Comfortcooling systems including room airconditioners, residential central airconditioners and air conditioning systemsfor ofce and commercial buildings areoccasionally used in these applications. Thisis usually because the differences betweencomfort and precision cooling systems canbe ambiguous.

Comfort cooling systems are engineeredprimarily for the intermittent use requiredto maintain a comfortable environmentfor people in facilities with a moderateamount of in-and-out trafc. However, whilethese systems are capable of effectivelymaintaining acceptable conditions for

human occupants, they are not designed toregulate humidity and temperature withinprecise margins.

Precision cooling systems, on the otherhand, are engineered primarily for facilitiesthat require year-round constant cooling,precise humidity control and a highercooling capacity per square foot. This typeof environmental conditioning can utilize a

wide range of cooling mediums, includingair, chilled water and pumped refrigerants.

To further elaborate, a number of factorsshould be taken into consideration whencomparing precision and comfort coolingsystems.

Cooling Optimized to ElectronicSystem Requirements

In recent years, there have been many

changes in data center cooling requirementsand capabilities that affect operation.Primary among these is an increase inthe density of new IT equipment and theresulting increase in heat loads. Higher heatdensities created by increased equipmentloads generate sensible heat a drierheat than that found in traditional facilityenvironments signicantly changing thedemands faced by the cooling system. Tobetter understand these demands, it isimportant to remember that there are twotypes of cooling: latent and sensible.

Latent cooling, in short, is the ability toremove moisture from the surroundingenvironment. This is an importantcomponent in typical comfort-coolingapplications such as ofce buildings, retailstores and other facilities with high humanoccupancy and use which are designedto maintain a comfortable balance oftemperature and humidity.

Sensible cooling, on the other hand, is theability to remove heat that can strictly bemeasured by a thermometer.


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Comfort cooling systems have a sensibleheat ratio of 0.60 to 0.70. This means thatthey are 60 to 70 percent dedicated tolowering temperature, and 30 to 40 percentdedicated to lowering humidity. Thesesystems are typically found in facilities with

considerable occupant trafc and doorsleading directly to the outside.

Conversely, precision cooling systems aredesigned to achieve a sensible heat ratio of0.80 to 1.0, with 80 to 100 percent of theireffort is devoted to lowering temperatureand only zero to 20 percent to loweringhumidity.

Unlike facilities that need to take occupantcomfort into consideration, data center

environments particularly network closetsand IDF/MDF rooms are typically notoccupied by people.

In most cases, they have limited accessand no direct means of egress to theoutside of the building except for seldom-used emergency exits. And because theirenvironment is primarily composed of dryheat, data centers require minimal moisture

removal.

In light of their unique environmentalattributes, most data center environmentsrequire a 0.80 to 0.90 sensible heat ratiofor effective and efcient cooling. This isbecause the need for latent heat removaldecreases in tandem with the need fordehumidication. This means more nominal20-ton comfort cooling units would berequired to handle the same sensible load asnominal 20-ton precision units.

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Figure 1. General office environments have loads that are 60-70 percent sensible (.6-.7SHR), while computer rooms have 90-100 percent sensible loads (.85-1.0 SHR)

Sensible Heat Ratio

General Office Computer Room

.65

1.0

SENSIBLE SENSIBLE

LATENT

LATENT .901.0


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Systems Sized to the Higher Densitiesof Data Center Environments

In a typical data center environment, heatdensities can be up to ve times higher thanin a typical ofce setting. And with high

density rack congurations becoming morepopular due to efcient virtualization, datacenter temperatures are increasing at afaster rate than ever before.

To illustrate, one ton of comfort coolingcapacity (12,000 BTU/hour, or 3,517 W) isrequired per 250-300 square feet of ofcespace. This translates into 12 to 14 watts persquare foot. In contrast, one ton of precisioncooling capacity is required per 50-100square feet of data center space.

That translates to a 35 to 70 watts persquare foot on average, with performanceof precision cooling continuing to increaseannually as new, more efcient systems aredeveloped. With this in mind, consider thatsome sites can have load densities as high as

200 to 300 watts per square foot.

From an airow standpoint, precisioncooling systems are designed to managelarger load densities than most comfortcooling systems. Precision coolingequipment typically supplies between 500and 900 cubic feet per minute (CFM) percooling ton.

This contrasts with the much smaller rangeof 350 to 400 CFM per cooling ton typically

delivered by comfort cooling equipment.

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Figure 2. Heat densities are typically three to five times higher in computer room environmentsthan in office environments.


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In addition, the use of blade servers andother newer rack-mounted equipment hascreated unprecedented new data centercooling requirements. New servers andcommunications switches generate heatdensities as much as ten times heat per

square foot as systems manufactured justten years ago. Comfort cooling systemsare simply ill-equipped to deal effectivelywith this new challenge and the associatedcooling required. Data center designersand engineers should consider the needfor specialized cooling units to work inconjunction with raised oor precisioncooling systems.

Furthermore, data centers using traditionalprecision cooling congurations with largecomputer room air conditioning (CRAC)units feeding air underneath the raised oorcan run into problems such a hot spots as high-density rack congurations areimplemented.

Even though there may be a high level ofcooling capacity in the room, the way air isbeing distributed can lead to problems if acooling unit fails. To mitigate these issues,critical cooling systems are integrating newtechniques such as row-based cooling,cold-aisle containment and other targetedcooling strategies. Further, emergingtechnologies in environmental control areallowing individual room-based CRAC unitsto be networked together and monitored

by comprehensive management programs.

Precise Humidity Control

Since most data centers operate 24 x 7 x365, precise humidity conditions must bemaintained around the clock. Ignoring theimpact of humidity can result in seriouslong-term problems, including damage toequipment and other resources, and to thefacilitys infrastructure.

The optimal dew point range for datacenters is 41.9 to 59 degrees Fahrenheitas recommended by the 2008 ASHRAEEnvironmental Guidelines for DatacomEquipment .1For example, above-normalmoisture can corrode switching circuitry,

causing malfunctions and equipmentfailures.

At the other end of the spectrum, lowhumidity can cause static discharges thatinterfere with normal equipment operation.This is a more likely scenario in a data center,since it is typically cooled 24 x 7, creatinglower levels of relative humidity.

Comfort cooling systems typically do nothave built-in humidity controls, making

it difcult to maintain stable dew pointlevels over extended periods of time. If thenecessary controls and components areadded, they must be specially congured tooperate as a complete system.

Precision cooling systems feature multi-mode operation to provide a properratio of cooling, humidication anddehumidication. This makes them muchmore suitable for the low tolerance range ofhumidity levels in a data center.

Protection Against AirborneContaminants

Even small amounts of dust or otherparticles can damage storage media andcharged electronic components. Mostcomfort cooling systems use residential-type air lters that are 10 percent efcient,making them inadequate for a data centerenvironment.

1It should be noted that the 2004 versionof these guidelines targeted a RH range of40 to 50 percent, rather than a specic dewpoint range.


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2MERV 8 or higher rated lters are requiredin HVAC equipment for the building to beeligible for LEED Certication

Precision cooling system lters have higherquality internal lter chambers that meetminimum MERV 8 ASHRAE requirements.2

Furthermore, systems that employ chilledwater and pumped refrigerant passively

lower temperatures through contact withsealed coils, further reducing the chance ofairborne particle contamination.

Efficient, Continuous Year-RoundCooling

Comfort cooling systems for most buildingsare designed to operate an average ofeight hours per day, ve days per week.This translates into about 1,200 hours peryear, assuming cooling is required only

during the summer months. Conversely,most data centers require cooling 24 hoursper day, 365 days per year regardless ofoutside weather conditions. Precisioncooling systems and their components arespecically engineered to meet this highcooling demand. An air-based precisioncooling units circulating fan is capable ofoperating continually, 8,760 hours per year,with other components turning on and offas needed. Furthermore, a precision systemcan operate effectively to minus 30 degreesFahrenheit, and Glycol cooled models cancool effectively down to minus 60 degreesFahrenheit. Comfort cooling systems withoutdoor heat exchangers, on the otherhand, are typically inoperable when outsideambient temperatures drop below 32degrees Fahrenheit due to lack of headpressure control.

Unique energy-saving congurations such as dual cooling systems also arebecoming more popular as a way to saveenergy. Utilizing this option, a conventionalair cooled direct expansion unit is convertedto a dual source cooling system by theaddition of a second coil that utilizes a

central building chiller supply. The unit canfunction either as a chilled water system, acompressor-based system, or a combinationof both. During times when the chillersupply is available, compressor operation iseliminated, reducing energy costs.

For colder climates, an even more effectiveenergy savings solution is to utilize anoutdoor uid cooler (drycooler and pumppackage) in conjunction with the chilledwater coil conversion, which will providefree cooling when ambient temperaturespermit.

Lower Operating Costs

Because of basic engineering, design and

equipment differences, a purchase pricecomparison of comfort versus precisionair conditioning systems does not tellthe complete story. A more accuratecomparison considers the difference inoperating costs between the two systems.

To follow is a basic example of an operatingcost comparison between the twoapproaches, using the assumptions outlinedbelow:

Each ton of cooling requires 1.0horsepower (or 0.747 kW)

The compressor motors and fans are 90percent efcient

Electricity costs $0.10 per kilowatt-hour

Humidication is required Novemberthrough March (3,650 hours)

The precision cooling system has a SHRof 0.90; the comfort system has a SHR of0.60.


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A more detailed analysis can be done onspecic equipment.

First, calculate the cost per ton of cooling fora year:

This results in a cost of $727 ton/yr.

Then, determine the cost per sensible ton ofcooling by dividing the total cost by the SHRfor each system. For the precision coolingsystem the cost per sensible ton is:

For the comfort cooling system, the cost persensible ton is:

In this example, the operating cost to runa comfort cooling system for one yearexceeds the cost to run a precision coolingsystem by $404 per ton of sensible load. Thisis consistent with the generally acceptedprinciple that it takes three tons of comfortcooling capacity to equal two tons ofprecision capacity.

A second point of comparison is the costof humidication, which is determined by

calculating the latent cooling that occurs perton of sensible cooling.

For a precision system:

For a comfort system:

The comfort system expends 6,667 BTU ofenergy per ton of sensible cooling to removehumidity that must be replaced to maintainrequired data center moisture content of45-50 percent.

The added cost is:

In this scenario, when all cooling andhumidication costs are considered, theoperating cost of a comfort-based systemexceeds the operating cost of a precisioncooling system by $1,119 per ton ofsensible cooling annually.

Enhanced Service and Support

Todays critical data center environmentsrequire advanced environmental supportsystems to efciently achieve business-critical continuity. Because precision coolingsystems run continuously and are necessaryfor the proper operation of IT equipment,regular maintenance is of the utmostimportance.

A precision cooling system cannot fullybenet the data center if it is not functioningat the highest level. Precision coolingsystems must be properly maintained andserviced throughout their entire lifecycleto maximize the organizations return oninvestment.

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6,667BTU/ton x 3,650hrs./yr. x $0.10/kWh

3,413 BTU/hr./kW= $713 ton/y

$727

0.60 SHR= $1,212 ton/yr.

$727

0.90 SHR= $808 ton/yr.

12,000 BTU/ton

0.60 SHR 12,000 BTU/ton= 8,000 latent BTU/ton

12,000 BTU/ton

0.90 SHR 12,000 BTU/ton= 1,333 latent BTU/ton

0.747 kW/ton x 8760 hrs./yr. x $0.10/kWh

0.90 efficiency


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The level of knowledge required to maintainstandard building comfort cooling systemsis very different from that required forprecision cooling systems.

Because precision cooling systems are

engineered and tuned very differentlyfrom comfort cooling systems, serviceproviders without specic knowledge ofhow to maintain the equipment may not beprepared to handle the job.

Inexperienced service providers greatlyincrease the risk of equipment failure,whereas OEM factory-trained serviceproviders can ensure that a precision coolingsystem is operating at its optimal point ofperformance and efciency.

Understanding these systems addedcomplexity, most precision cooling systemmanufacturers employ factory-trained,locally-based installation, service andsupport partners that are accustomedto the needs and sensitivities of workingwith specic equipment in the data centerenvironment.

While non-OEM service providers may havethe knowledge required to install precision

cooling equipment, they do not necessarilyhave the service expertise needed to assureoptimum performance of these systemsthroughout their lifecycles. Manufacturer-backed service technicians are trained bythe factory to handle all aspects of serviceand have the knowledge and experiencenecessary to pinpoint subtle areas of concernwithin the data center that may be causingissues with the cooling units.

Because even a short amount of downtimecan impact an organizations bottom line,many precision air systems are designedfor rapid serviceability and, in most cases,are backed by factory-trained servicepersonnel who can arrive in nearly half the

time required by other providers. Dependingupon the manufacturer, 24-hour emergencyservice and preventive maintenance servicemay be available.


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Conclusion

Business-critical data centers have unique environmental requirements and, as a result,necessitate cooling systems that match those requirements.

While comfort cooling systems are appropriate for comfort environments facilities that

are occupied by people or that house routine equipment and supplies they are not wellsuited to environments that require precise regulation of temperature, humidity and airquality. Furthermore, though initial equipment costs may favor the installation of comfortcooling equipment, energy costs over the life of the equipment and overall cooling qualitymake precision cooling equipment the better long-term investment.

Precision cooling systems are specically designed to address the unique demands oftodays complex IT environments. When maintained properly, these systems can meet theincreasing demands for heat rejection, humidity control, ltration and other requirementsin data centers and other high availability computer facilities while also providing additionalefciency, reliability and exibility benets.


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While every precaution has been taken to ensure accuracy andcompleteness in this literature, Liebert Corporation assumes noresponsibility, and disclaims all liability for damages resultingfrom use of this information or for any errors or omissions.

2010 Liebert Corporation. All rights reserved throughoutthe world. Specifications subject to change without notice.All names referred to are trademarks or registered trademarksof their respective owners.Liebert and the Liebert logo are registered trademarks of theLiebert Corporation. Business-Critical Continuity, Emerson NetwPower and the Emerson Network Power logo are trademarks anservice marks of Emerson Electric Co. 2010 Emerson Electric C

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